(a) Field of the Invention
The present invention relates to an organic thin film electroluminescence (EL) display unit and, more particularly, to a two-dimensional organic thin film EL display unit capable of being driven by a pair of electrodes.
(b) Description of the Related Art
Some organic thin films are known to have an EL effect when the organic thin films are interposed between two electrodes, i.e., between an anode and a cathode. In these organic thin films, holes and electrons injected from the electrodes are recombined to provide an energy for generating luminescence based on the EL phenomenon. This EL phenomenon is generally called "organic thin film EL" and being developed by active researches for a display unit because of its higher energy efficiency and a lower voltage driving capability, as compared to other display devices. The organic EL display unit can be driven by a direct current as low as at several volts to several tens of volts.
The organic thin film EL phenomenon can be observed even in a single organic EL thin film. To obtain a high energy luminance at a lower applied voltage, it is desirable to improve the injection efficiency of the carriers injected into the organic EL film from each electrode. For this purpose, a multilayer structure including a carrier injection layer or carrier transport layer interposed between the electrode and organic EL thin film is proposed in some literatures, in order to reduce the energy barrier between the electrode and the organic EL thin film to thereby enhance mobility of the carriers between the electrode and the organic EL thin film.
Examples of the literatures include: Patent Publication No. JP-A-1982-51781 which proposes a multilayer structure of anode/organic hole transport layer/organic EL thin film/cathode; Appl. Phys. Lett. 55, 1489 (1989) presented by Adachi et al. and entitled Organic electroluminescent device having a hole conductor as an emitting layer which proposes another multilayer structure of anode/organic EL thin film/organic electron transport layer/cathode; and Patent Publication No. JP-A-1994-314594 which proposes another multilayer structure of anode/a plurality of organic hole injection and transport layers/organic EL thin film/a plurality of organic electron injection and transport layers/cathode. The order of the films in these multilayer structures may be reversed from the recited orders.
FIG. 1 shows one of the conventional multilayer structures as mentioned above, which includes anode 41/organic hole transport layer 42/organic EL thin film 43/cathode 44 consecutively formed on a supporting substrate 40. Materials for at least one of the electrodes should be transparent for emission of the generated luminescence, and indium-tin-oxide (ITO) is generally used as the material for the anode 41 for this purpose. Materials for the cathode 44 is selected from metals such as Mg, Al and In, which have a low work function for reducing barrier for the injected electrons, and which may or may not be doped with Ag or Li, as described in Patent Publication No. JP-A-1993-121172, for example.
Materials for the organic hole transport layer 42 can be selected from aromatic tertiary amine, porphyrin derivatives etc. and material for the organic EL thin film 43 can be selected from 8-hydroxy quinoline metallic complex, butadiene derivatives, coumalin derivatives, benzoxazole derivatives, oxadiazole derivatives, oxazole derivatives, thiadiazole derivatives, styreneamine derivatives, bisstyrylbenzen derivatives, bis-styrylanthracene derivatives, perylene derivatives, aminopyrene derivatives, etc. Material for the organic electron transport layer, if provided, can be selected from naphtalimide derivatives, perylene tetracarbon acid diimide derivatives, quinacridone derivatives etc. The electrodes 41 and 44 and organic thin films 42 and 43 are consecutively formed on the supporting substrate 40, such as glass or resin film, by a dry deposition method, such as vacuum evaporation or sputtering, or a wet deposition method, such as spin-coating or dipping in a solution wherein the above mentioned materials are dispersed or dissolved in a resin or solvent. When a transparent electrode is formed as a first layer electrode 41, the supporting substrate 40 should be also transparent for emission of the luminescence.
A two-dimensional luminescent display unit using the organic thin film EL is proposed wherein a plurality of unit pixels or EL cells each formed by the organic thin film EL multilayer structure as described above are arranged on a supporting substrate in a two-dimensional array (see, for example, Patent Publication No. JP-A-1995-78690). FIG. 2 shows the proposed two-dimensional display unit, wherein a plurality of first layer stripe electrodes 41a extending parallel to one another, an organic multilayer structure 43b including at least one organic EL thin film and a plurality of second layer stripe electrodes 44a extending perpendicular to the first layer stripe electrodes 41a and parallel to one another are consecutively formed on a supporting substrate 40. The crossing portions of the first layer electrodes 41a and the second layer electrodes 44a constitute EL cells arranged in a two-dimensional array and driven by the pair of electrode groups 41a and 44a in a matrix drive.
The two-dimensional EL display unit as described above, however, suffers from degradation in the image quality when the EL cells have scattering or variation of luminescence or defects in some cells because the luminescence are generally observed from the front to thereby emphasize the defects. In addition, since one of the first and second layer electrode groups 41a and 44a is transparent and the other is metallic, the EL device has a poor contrast due to the reflection of external light by the metallic electrodes in the cells which are not luminous at that instant, the metallic electrodes being disposed behind the screen.
Further, patterning by use of a known photolithographic technique is difficult to apply to the organic EL materials because the EL materials have generally insufficient resistance to acidic or alkali solution or organic solvent, which are essential to the process in fabrication of the EL display unit. Especially, for a color display unit, a variety of organic EL materials are used for imaging different colors, some of which may have insufficient resistance. Patent Publication No. JP-A-1995-142169 proposes the solution for this problem, wherein a multilayer structure including organic EL thin films for generating three primary colors is interposed between the anode and cathode to generate white luminescence, which is then filtered by a color filter such as used in a liquid crystal display device. The solution, however, has disadvantages in that the drive voltage requested for driving the organic EL thin film generally rises due to the large thickness of the multilayer structure, that a shutter mechanism is required for the filter, and that EL energy is absorbed by the inserted color filter.
Patent Publication No. JP-A-1981-62284 describes another two-dimensional organic EL display unit wherein a plurality of one-dimensional EL arrays each disposed on a stripe supporting substrate are arranged by adhesion to obtain a two-dimensional display unit. In the proposed display unit, however, the display unit has similar defects of degradation in the image quality because of the scattering in the luminescence. Further, both the latter display units have similar defects of a low brightness in the screen because of the small cell area, especially in the case of a fine cell.